1. Field of the Invention
The invention relates to a planar polarographic sensor.
2. Background Information
Known polarographic sensors serve to determine the lambda value of gas mixtures which defines the ratio of the total oxygen to the oxygen contained in the fuel-air mixture that is combusted in a cylinder, with the sensors determining the oxygen content of the exhaust gas via a change in the boundary flow. Due to cost-effective production methods, the manufacture of planar polarographic sensors that may be produced in ceramic-sheet and screen-print technology has become popular in practice in recent years. These planar polarographic sensors may be produced in a simple and economic manner based on the sheet-like oxygen conducting solid electrolyte bodies made of, for example, zirconium oxide and each coated on both sides with an inner and outer pumping electrode including associated conductors. The inner pumping electrode in this case is disposed in an advantageous manner in the border region of a diffusion channel through which the measuring gas is fed and which serves as gas diffusion resistance.
To improve the reproducibility of the diffusion resistance, DE-OS 38 11 713 proposes to form the diffusion resistance by means of a porous sintering form body without an air gap which is inserted into the unsintered sensor. To accomplish this, a recess is punched out of a solid electrolye sheet and the porous form body is then inserted into the same. In order to supply the measuring gas, a diffusion channel is either guided transversely through the sensor layers to the porous form body or the porous form body is exposed at the end face of the sensor.
EP-A-01 94 082 discloses a planar polarographic sensor whose diffusion resistance is formed by a gap extending parallel to the solid electrolyte body and by a porous insulating layer disposed opposite the gas and covering the electrode. The measuring gas is supplied to the electrode via the gas and the porous insulating layer. The measured gas is supplied to the electrode via the gap and the porous insulating layer.
The production of known planar polarographic sensors is expensive. For example, in order to set the diffusion hole, a punching process is required whose positioning precision is of special significance for the sensor to function. Configuring the diffusion zone with an air gap, on the other hand, requires additional screen printing steps. Each additional screen printing step means an additional drying step and thus increases the chance of altering the length of the substrate. This, in turn, has a negative effect on the positioning precision of the subsequent screen printing steps, which has an adverse effect on reproducibility, especially if a sensor is widened with Nernst cells.